1. Field of the Invention
The present invention relates to a paper supply unit which supplies stacked sheet-like recording media, sheet by sheet, and an image forming apparatus including the paper supply unit.
2. Description of the Related Art
In an image forming apparatus such as a printer, a copier, a facsimile machine, or a multifunction peripheral (MFP) having functions of at least two thereof, a paper supply unit is provided which supplies sheets of paper as recording media stacked on a paper supply cassette or a paper supply tray, sheet by sheet. As such a kind of a paper supply unit, a paper supply unit of a roller paper supply type is known which sends out sheets of paper stacked on a paper supply tray to a conveyance path using a pickup roller.
In the paper supply unit of the roller paper supply type, it is necessary to press sheets of paper to the pickup roller with certain pressure, and the paper supply pressure is generated as a result of a bottom plate on which the sheets of paper are placed being pressed upward by a spring. At this time, the necessary paper supply pressure differs depending on each particular type of the paper. Generally speaking, the paper supply pressure is to be increased as the thickness of a sheet of paper becomes greater, and in the contrary, the paper supply pressure is to be decreased as the thickness of a sheet of paper becomes less. Then, when the paper supply pressure is too high with respect to proper one, supplying plural sheets of paper at a time occurs, and when the paper supply pressure is too low, a failure of supplying a sheet of paper occurs. The paper supply pressure of a normal image forming apparatus is designed to have a value corresponding to a thickness of plain paper that is commonly used.
However, there is a user who intends to use mainly thin paper or thick paper which thus needs the paper supply pressure outside the range of the paper supply pressure of the paper supply unit. When such a kind of paper is supplied, the normal paper supply pressure is to be changed to correspond to the kind of paper since supplying plural sheets of paper at a time or a failure of supplying a sheet of paper occurs when the normal paper supply pressure is used.
Japanese Laid-Open Patent Application No. 6-127710 (hereinafter, simply referred to as Patent Document 1) discloses a cassette paper supply unit in which it is possible to adjust the paper supply pressure. However, according to Patent Document 1, complicated work may be needed to change the paper supply pressure, and much time may be consumed therefor. That is, according to Patent Document 1, in a first embodiment, a support for a pressure spring that generates the paper supply pressure is removed, and then, the support for the pressure spring is fitted to another position. In this method, since the paper supply pressure changing work is carried out after the pressure spring is thus completely removed from the paper supply cassette, time may be consumed.
According to Patent Document 1, in a second embodiment, as a result of an operator rotating a pressure spring, the pressure spring is gradually embedded into a cylindrical depression provided below a bottom plate of a paper supply unit. In this method, the cylindrical depression has a spiral groove on an inner circumferential wall, and a spiral element of the pressure spring is gradually fitted into the spiral groove of the cylindrical depression as the pressure spring is rotated by the operator. Thus, the length of an upper part of the pressure spring projecting from the top end of the cylindrical depression can be adjusted merely by rotating the pressure spring to adjust the length of a lower part of the pressure spring embedded in the cylindrical depression. However, in this method, the operator needs to remove the bottom plate from a body of the paper supply unit in order to rotate the pressure spring that presses the bottom plate to apply the paper supply pressure to the paper placed on the bottom plate. Thus, in this method, removing the bottom plate, and then returning the same (after the adjustment of the paper supply pressure by rotating the pressure spring) may consume much time.
Further, in the paper supply unit according to Patent Document 1, in addition to the above-mentioned problems concerning the time consumed for the work, the pressure spring is in a state of always lifting the bottom plate toward a pickup roller. Therefore, it may be difficult to set sheets of paper in the paper supply cassette, and the number of sheets of paper that can be set in the paper supply cassette may be reduced.